1. Field of the Invention
The invention relates to a process for positioning a mask relative to a workpiece and a device for executing the process in an exposure device which is used for production of a semiconductor device, a printed board, an LCD (liquid crystal display) and for similar purposes.
2. Description of Related Art
Production of electrical and electronic components and parts of various types in which processing of structures in the micron range is necessary includes an exposure process. These electronic parts are semiconductor components, liquid crystal displays, printer heads of the inkjet type, and multichip modules in which a host of different electronic components are produced on a substrate and thus a module is formed, and the like.
In this exposure process, it is important, in the case of transmission of the mask pattern onto the workpiece, that a pattern to be subsequently transferred is positioned exactly to a pattern formed beforehand.
The above described positioning is conventionally done such that the alignment marks of the mask and the workpiece come to lie on top of one another.
As automated devices for transferring a mask pattern, exposure devices of the projection type are known wherein positioning is done using exposure light, for example of an i-line, an h-line or a g-line (i-line: 365 nm wavelength, h-line: 405 nm wavelength, g-line: 436 nm wavelength), or wherein positioning is done using nonexposure light, as, for example, an e-line, a d-line or a c-line (e-line: 546 nm wavelength, d-line: 588 nm wavelength, c-line: 656 nm wavelength).
In the aforementioned devices, in the area used for positioning, a circuit pattern cannot be formed since the photoresist is exposed to the action of exposure light during positioning.
In the case in which the yield from a workpiece is to be a host of chips, as in semiconductor components, the number of chips decreases according to the area used for positioning. Therefore, the disadvantage arises that production efficiency is impaired. On the other hand, in these devices, there is the advantage that the region of the circuit patterns is not limited by the positioning region, since the photoresist is not exposed to the action of exposure light during positioning.
However, it is considered disadvantageous that, as a result of the different wavelengths of the exposure light and the nonexposure light, the mask pattern is not projected onto the same site.
FIG. 11 shows, schematically, an arrangement of the conventional exposure device in which positioning is done using the above described e-line. In the drawing reference number 1 indicates an exposure light irradiation device or a nonexposure light irradiation device; reference number 1a, a lamp; reference number 1b, a focussing mirror; reference number 1c, a shutter; reference number 1d, an optical filter; and reference number 1e, a condenser lens.
Furthermore, reference number 2 indicates a mask carrier on which mask M is attached by means of a vacuum chuck or the like and which is driven by means of a drive device not shown in the drawing in the X-Y-Z-directions (X axis and Y axis being orthogonal axes on the plane parallel to one mask surface; Z axis being the axis in the vertical direction in FIG. 11; .theta. axis being the axis of rotation around the Z-axis).
On mask M, a mask pattern and mask alignment marks MAM, hereinafter referenced to as mask marks, are recorded for purposes of positioning.
Reference number 3 indicates a projection lens and reference letter W indicates a workpiece. Workpiece alignment marks WAM, hereinafter referenced to as workpiece marks, are recorded on workpiece W for purposes of positioning.
Reference number 5 indicates an alignment unit which consists of lens 5a, objective lens 5b, half mirror 5c and image converter 5d which has a CCD camera. Workpiece mark WAM and mask mark MAM, which is projected onto the workpiece W, are detected via half mirror 5c, objective lens 5b and lens 5a by means of image converter 5d, and thus the positions of the two marks are observed.
In FIG. 11, alignment unit 5 is shown only once. However, there are several alignment units 5 (at least at two locations) since there are several mask marks MAM and several workpiece marks WAM (each at least at two locations) each on mask M and workpiece W, and because alignment unit 5 is assigned according to the respective alignment mark.
Furthermore, alignment units 5 are ordinarily formed such that they can be removed in the direction of the arrow according to FIG. 11. During exposure, alignment units 5 are removed when they extend within the exposure area.
Alignment units 5 can be located in area B as shown. Moreover, they can also be located in additionally shown area A or area C.
In FIG. 11 when workpiece W is exposed, the e-line, as nonexposure light, is emitted first from exposure light (or nonexposure light) irradiation device 1 via optical filter 1d and, by means of alignment units 5, workpiece marks WAM and mask marks MAM, projected onto workpiece W, are detected. This method of determining mask marks MAM projected onto the workpiece by the projection lens is called TTL, i.e., the "through the lens", method.
In the case of auto alignment, by means of a control device which is not shown in the drawing, based on the features of mask marks MAM and workpiece marks WAM, the respective marks are recognized and mask carrier 2 and/or a workpiece carrier 4 are automatically moved such that the positions of the two marks agree with one another.
In the case of manual alignment, detected mask marks MAM and workpiece marks WAM are displayed on a monitor, which is not shown in the drawing, and by watching the monitor, an operator moves mask carrier 2 and/or workpiece carrier 4 such that the two marks agree with one another.
After completed positioning, optical filter 1d is removed from the optical path, the i-line, as exposure light, is then emitted from exposure light (or nonexposure light) irradiation device 1, the mask pattern is projected onto workpiece W, and exposure is effected.
In the case of using the i-line as the exposure light and the e-line as the nonexposure light, as a result of the different wavelengths of the two lines, the following disadvantages arise in the above described positioning:
1) In a projection lens which is designed such that at exposure wavelengths no imaging error or aberration occurs, at nonexposure wavelengths, a deviation of the mask pattern projection surface takes place. This means that the mask pattern is not projected onto the same surface as is illustrated in FIG. 11 wherein the pattern is projected on surfaces I and E.
2) The positions of mask marks MAM' on mask pattern projection surface E by the e-line, and the positions of mask marks MAM on mask pattern projection surface I by the i-line, do not agree with one another as a result of the chromatic aberration of the magnification factor.
This means that the distance between projection lens 3 and mask pattern projection surface E by the e-line becomes greater than the distance between projection lens 3 and mask pattern projection surface I by the i-line, and distance Le of mask marks MAM' on mask pattern projection surface E by the e-line becomes greater than distance Li of mask marks MAM on mask pattern projection surface I by the i-line, as shown in FIG. 11.
Furthermore, the positions of the projection images of mask marks MAM' on mask pattern projection surface E are changed by the locations of the alignment marks. This means that different values are obtained for distances .DELTA.L and .DELTA.L' between the workpiece marks and projected mask marks, as illustrated in FIG. 12.
Conventionally, therefore, the deviation of the projection surfaces is corrected by inserting a parallel flat plate in the optical path of the projection optics system and, according to the wavelengths of the nonexposure light, by changing the plate thickness or adjusting the plate tilt (JP patent HEI 5-43168 and corresponding U.S. Pat. No. 4,616,130).
In the above described prior art, the deviation of the projection images by the exposure light and the nonexposure light is corrected by inserting the parallel flat plate between the projection lens and the workpiece. The property being that, by inserting the parallel flat plate in the optical path, the focal position deviates by .DELTA.S, as is illustrated in FIG. 13.
The above described prior art, however, has the following disadvantages:
(1) As is shown in FIG. 12, the positions of the projection images of the mask marks are changed by the nonexposure light depending on the positions of the alignment marks. It is therefore necessary to insert the parallel flat plate between the mask and the projection lens, to adjust its tilt and to correct the position deviation by the positions of the alignment marks, as is shown, for example, using the conventional device.
(2) When the wavelength for exposure and the wavelength for alignment change, the amount of focal length correction changes. It is therefore necessary to make available the parallel flat plate according to a combination of the above described wavelengths.
(3) To adjust the magnification factor, an optical component is used which consists of the parallel flat plate. It is therefore necessary to additionally use a retaining device for this purpose. Especially in the case in which the optical component is inserted and removed again, does the disadvantage arise that the device becomes complex and too large at the same time.
(4) The disadvantage arises that the workpiece is contaminated by dust because directly above the workpiece is the insertion device of the parallel flat plate.
As was described above, positioning using the nonexposure light has the advantage that the pattern can be formed in the vicinity of the alignment marks. But then it is necessary to correct the length of the optical path using the parallel flat plate and the like. This results in various disadvantages.
Furthermore, in the conventional process in which the relative positions of the two alignment marks are determined in the state in which the mask mark is projected onto the workpiece provided with the workpiece marks, the disadvantage arises that when auto alignment is done, the control device only recognizes the workpiece marks with difficulty.
To eliminate the above described disadvantages, the applicant has already proposed the device shown in FIG. 14 (JP patent application HEI 6-294279 and corresponding U.S. Pat. application No. 08/564,005).
In the drawing, the same parts as in FIG. 11 are provided with the same reference numbers as in FIG. 11. Reference number 1 indicates the exposure light (or nonexposure light) irradiation device, reference number 2 indicates the mask carrier, reference letter M indicates the mask on which a mask pattern and the mask alignment marks MAM are recorded for positioning. Reference number 3 indicates the projection lens and reference letter W indicates the workpiece on which workpiece marks WAM are recorded for purposes of positioning. Furthermore, reference number 4 indicates the workpiece carrier on which total reflection or half mirror 4a is installed and which is driven by means of a drive device which is not shown in the drawing in the X-Y-Z-.theta. directions.
Reference symbol WA1 indicates an alignment mark partial illumination system. The nonexposure light which is emitted from a light source not shown in the drawing is incident via optical fibers 6a, lens 6b and mirror 6c on half mirror 5e of alignment unit 5 and irradiates workpiece mark WAM on workpiece W.
Exposure will now be described using the device shown in FIG. 14:
(1) Mask M is placed on mask carrier 2 and attached. Shutter 1c of exposure light (or nonexposure light) irradiation device 1 is opened in the state in which optical filter 1d in the optical path is removed. Exposure light is emitted onto mask M. In this process, step workpiece W is not placed on workpiece carrier 4.
(2) The position of workpiece carrier 4 is shifted in the Z-direction such that the reflection surface of mirror 4a installed in workpiece carrier 4 agrees with mask pattern projection surface Zo by the exposure light.
(3) Alignment unit 5 is inserted. Alignment mark partial illumination system WA1 is arranged integrally with alignment unit 5 and is inserted together with alignment unit 5.
(4) By means of image converter 5d of alignment unit 5, mask mark MAM, projected onto mirror 4a of workpiece carrier 4, is detected. An image processing part, not shown in the drawing, identifies mask mark MAM based on the detected image and stores the position coordinates thereof (XM, YM).
(5) Shutter 1c of exposure light (or nonexposure light) irradiation device 1 is closed. In this way emission of exposure light onto mask M is stopped.
(6) Workpiece W is placed on workpiece carrier 4 and attached.
(7) Workpiece carrier 4 is moved in the Z-axis direction such that the surface of workpiece W agrees with mask pattern projection surface Zo.
(8) Illumination light is supplied to alignment mark partial illumination system WA1. Workpiece mark WAM on workpiece W is irradiated via optical fibers 6a, lens 6b, mirror 6c, half mirror 5e, lens 5b and half mirror 5c, and is detected by means of image converter 5d.
The image of workpiece mark WAM, detected by means of image converter 5d, is sent to the above described image processing part which identifies workpiece mark WAM and determines its position coordinates (XW, YW).
(9) Emission of the illumination light of alignment mark partial illumination system WA1 is stopped.
(10) The position deviation of workpiece W and mask M is determined on the basis of stored position coordinates (XM, YM) of mask mark MAM and determined position coordinates (XW, YW) of the workpiece mark.
As a result of the above described position deviation, mask carrier 2 and/or workpiece carrier 4 is/are driven in the X-Y-Z-.theta. directions and the position of mask mark MAM is brought into agreement with the position of workpiece mark WAM.
With the above described agreement of the position of mask mark MAM with the position of workpiece mark WAM, alignment unit 5 is removed. Shutter 1c of exposure light (nonexposure light) irradiation device 1 is opened, the exposure light is emitted onto mask M, and thus exposure is effected.
In the case of using the above described exposure for a step and repeat process in which each exposure zone on the workpiece is exposed in steps by moving the workpiece, according to above described process (10), the following processes are also performed, the processes of (8) through (15) are repeated, and exposure of the respective exposure zone is effected.
(11) Alignment unit 5 is removed.
(12) Shutter 1c of exposure light (nonexposure light) irradiation device 1 is opened, the exposure light is emitted onto mask M, and thus exposure is effected.
(13) Shutter 1c of exposure light (nonexposure light) irradiation device 1 is closed, and emission of exposure light is stopped.
(14) Workpiece carrier 4 is moved in a certain amount in order that the next exposure zone is positioned in a stipulated exposure position.
(15) Alignment unit 5 is inserted. There is a return to above described process (8). Processes (8) through (15) are repeated and the respective exposure zone on the workpiece undergoes step and repeat exposure.
The above described technique suggested previously by the applicant does have the advantages that no parallel flat plate is used and that positioning of the mask to the workpiece can be done independently of the projection lens and with high precision. In the case of its use for the above described step and repeat exposure, however, the following disadvantage arises:
In the case of using the previously suggested exposure process for step and repeat exposure, it is necessary to insert and remove alignment unit 5 when each exposure is effected. As a result, the reproducibility of the insertion position of alignment unit 5 is important.
FIG. 15 schematically shows a case in which the stopping position of alignment unit 5 changes. When the stopping position of alignment unit 5 is identical to the position in FIG. 15 by the broken line, the imaging position of mask mark MAM (or of workpiece mark WAM) on image converter 5d, changes accordingly.
In the above described step and repeat cycle of processes (8) through (15), in step and repeat exposure, only the positions of workpiece mark WAM are, however, detected and stored starting the second time. Therefore, in this case, it is possible that deviations occur from the position of mask mark MAM which was stored the first time.
In step and repeat exposure, the alignment accuracy is usually roughly .+-.1 micron. A positioning accuracy of alignment unit 5 of roughly .+-.0.1 microns (roughly 1/10 of the accuracy of alignment of mask M to workpiece W) is necessary in order that the deviations of workpiece marks WAM from the stored position of mask mark MAM lie within this range. It is difficult to do this mechanically with each insertion and removal of alignment unit 5.
The above described disadvantage is, on the other hand, eliminated by the position of mask mark MAM being stored in each step and repeat cycle. However, in the above described case, the sequence of storing the position of mask mark MAM by emission of exposure light cannot be done because, beginning with the second step and repeat cycle, workpiece W is attached to workpiece carrier 4.